Sequence and developmental expression of AmphiTob, an amphioxus homolog of vertebrate Tob in the PC3/BTG1/Tob family of tumor suppressor genes

BTG/Tob family members Tob1 and Tob2 inhibit proliferation of mouse embryonic stem cells via Id3 mRNA degradation

The mammalian BTG/Tob family is a group of proteins with anti-proliferative ability, and there are six members including BTG1, BTG2/PC3/Tis21, BTG3/ANA, BTG4/PC3B, Tob1/Tob and Tob2. Among them, Tob subfamily members, specifically Tob1/Tob and Tob2, have the most extensive C-terminal regions. As previously reported, overexpression of BTG/Tob proteins is associated with the inhibition of G1 to S-phase cell cycle progression and decreased cell proliferation in a variety of cell types. Tob subfamily proteins have similar anti-proliferative effects on cell cycle progression in cultured tumor cells. An important unresolved question is whether or not they have function in rapidly proliferating cells, such as embryonic stem cells (ESCs). Tob1 and Tob2 were expressed ubiquitously in mouse ESCs (mESCs), suggesting a possible role in early embryonic development and mESCs. To address the above question and explore the possible functions of the Tob subfamily in ESCs, we established ESCs from different genotypic knockout inner cell mass (ICM). We found that Tob1(-/-), Tob2(-/-), and Tob1/2 double knockout (DKO, Tob1(-/-) & Tob2(-/-)) ESCs grew faster than wild type (WT) ESCs without losing pluripotency, and we provide a possible mechanistic explanation for these observations: Tob1 and Tob2 inhibit the cell cycle via degradation of Id3 mRNA, which is a set of directly targeted genes of BMP4 signaling in mESCs that play critical roles in the maintenance of ESC properties. Together, our data suggest that BTG/Tob family protein Tob1 and Tob2 regulation cell proliferation does not compromise the basic properties of mESCs.

Down-regulation of BTG1 by miR-454-3p enhances cellular radiosensitivity in renal carcinoma cells

BACKGROUND

B cell translocation gene 1 (BTG1) has long been recognized as a tumor suppressor gene. Recent reports demonstrated that BTG1 plays an important role in progression of cell cycle and is involved in cellular response to stressors. However, the microRNAs mediated regulatory mechanism of BTG1 expression has not been reported so far. MicroRNAs can effectively influence tumor radiosensitivity by preventing cell cycle progression, resulting in enhancement of the cytotoxicity of radiotherapy efficacy. This study aimed to demonstrating the effects of microRNAs on the BTG1 expression and cellular radiosensitivity.

METHODS

The human renal carcinoma 786-O cells were treated with 5 Gy of X-rays. Expressions of BTG1 gene and miR-454-3p, which was predicted to target BTG1 by software algorithm, were analyzed by quantitative polymerase chain reaction. Protein expressions were assessed by Western blot. Luciferase assays were used to quantify the interaction between BTG1 3'-untranslated region (3'-UTR) and miR-454-3p. The radiosensitivity was quantified by the assay of cell viability, colony formation and caspase-3 activity.

RESULTS

The expression of the BTG1 gene in 786-O cells was significantly elevated after treatments with X-ray irradiation, DMSO, or serum starvation. The up-regulation of BTG1 after irradiation reduced cellular radiosensitivity as demonstrated by the enhanced cell viability and colony formation, as well as the repressed caspase-3 activity. In comparison, knock down of BTG1 by siRNA led to significantly enhanced cellular radiosensitivity. It was found that miR-454-3p can regulate the expression of BTG1 through a direct interaction with the 3'-UTR of BTG1 mRNA. Decreasing of its expression level correlates well with BTG1 up-regulation during X-ray irradiation. Particularly, we observed that over-expression of miR-454-3p by transfection inhibited the BTG1 expression and enhanced the radiosensitivity. In addition, cell cycle analysis showed that over-expression of miR-454-3p shifted the cell cycle arrest from G2/M phase to S phase.

CONCLUSIONS

Our results indicate that BTG1 is a direct target of miR-454-3p. Down-regulation of BTG1 by miR-454-3p renders tumor cells sensitive to radiation. These results may shed light on the potential application in tumor radiotherapy.

The role of antiproliferative gene Tob1 in the immune system

Tob1 (transducer of ERBB2-1, TOB1 is humans) is a member of the antiproliferative (APRO) family of proteins that controls cell cycle progression in several cell types. In addition, Tob1 has been implicated in diverse cellular mechanisms such as embryonic dorsal development, and T helper 17 (Th17) cell function. More recently, evidence linking Tob1 function to experimental and human immune related disorders has mounted, thus underscoring the potential of this molecule as a biomarker and as a therapeutic target. This article reviews these functions with an emphasis on their implications for human autoimmune diseases such as multiple sclerosis.

Opposing Nodal/Vg1 and BMP signals mediate axial patterning in embryos of the basal chordate amphioxus

The basal chordate amphioxus resembles vertebrates in having a dorsal, hollow nerve cord, a notochord and somites. However, it lacks extensive gene duplications, and its embryos are small and gastrulate by simple invagination. Here we demonstrate that Nodal/Vg1 signaling acts from early cleavage through the gastrula stage to specify and maintain dorsal/anterior development while, starting at the early gastrula stage, BMP signaling promotes ventral/posterior identity. Knockdown and gain-of-function experiments show that these pathways act in opposition to one another. Signaling by these pathways is modulated by dorsally and/or anteriorly expressed genes including Chordin, Cerberus, and Blimp1. Overexpression and/or reporter assays in Xenopus demonstrate that the functions of these proteins are conserved between amphioxus and vertebrates. Thus, a fundamental genetic mechanism for axial patterning involving opposing Nodal and BMP signaling is present in amphioxus and probably also in the common ancestor of amphioxus and vertebrates or even earlier in deuterostome evolution.

Tob, a member of the APRO family, regulates immunological quiescence and tumor suppression

Cellular quiescence is a state characterized by decreased cell size and metabolic activity. Quiescence acts to reduce the resources, energy and space. Quiescence might also protect cells from accumulating metabolic damage that could result in malignancy. Recent studies have shown that cell quiescence is an actively maintained rather than a default state in the absence of signals. Quiescence factors represent potential tumor suppressor genes because alterations in their expression or function contribute to progression of malignancies. There is growing evidence that quiescence is under active transcriptional control. The regulation of cell proliferation involves dozens of extracellular signals and intracellular factors of various types. In the present review we will focus on the role of Tob, a member of the APRO family members in regulating cellular quiescence and inhibition of cellular proliferation.

Btg1 and Btg2 gene expression during early chick development

Btg/Tob genes encode for a new family of proteins with antiproliferative functions, which are also able to stimulate cell differentiation. Btg1 and Btg2 are the most closely related members in terms of gene sequence. We analyzed their expression patterns in avian embryos by in situ hybridization, from embryonic day 1 to 3. Btg1 was distinctively expressed in the Hensen's node, the notochord, the cardiogenic mesoderm, the lens vesicle, and in the apical ectodermal ridge and mesenchyme of the limb buds. On the other hand, Btg2 expression domains included the neural plate border, presomitic mesoderm, trigeminal placode, and mesonephros. Both genes were commonly expressed in the myotome, epibranchial placodes, and dorsal neural tube. The results suggest that Btg1 and Btg2 are involved in multiple developmental processes. Overlapping expression of Btg1 and Btg2 may imply redundant functions, but unique expression patterns suggest also differential regulation and function.

Tob genes in development and homeostasis

Members of the Btg/Tob protein family share a conserved N-terminal region that confers the activity to inhibit cell proliferation. Tob1 and Tob2 proteins, which constitute a Tob subfamily, have a longer C-terminal region than BTG proteins. Apparently, genomes of invertebrates and teleost species contain only a single Tob locus, whereas genomes of mammalian, avian, and amphibian species contain two Tob loci (Tob1 and Tob2). Tob genes are expressed in oocytes, sperm, early embryos, and various adult tissues, depending on the species. Recent reports indicate that Tob proteins play important roles in spermatogenesis, embryonic dorsoventral patterning, osteogenesis, T-cell activation, and learning and memory. Accumulating evidence supports the hypothesis that Tob proteins act primarily as transcriptional repressors in several signaling pathways.

A gene catalogue of the amphioxus nervous system

The elaboration of extremely complex nervous systems is a major success of evolution. However, at the dawn of the post-genomic era, few data have helped yet to unravel how a nervous system develops and evolves to complexity. On the evolutionary road to vertebrates, amphioxus occupies a key position to tackle this exciting issue. Its “simple” nervous system basically consists of a dorsal nerve cord and a diffuse net of peripheral neurons, which contrasts greatly with the complexity of vertebrate nervous systems. Notwithstanding, increasing data on gene expression has faced up this simplicity by revealing a mounting level of cryptic complexity, with unexpected levels of neuronal diversity, organisation and regionalisation of the central and peripheral nervous systems. Furthermore, recent gene expression data also point to the high neurogenic potential of the epidermis of amphioxus, suggestive of a skin-brain track for the evolution of the vertebrate nervous system. Here I attempt to catalogue and synthesise current gene expression data in the amphioxus nervous system. From this global point of view, I suggest scenarios for the evolutionary origin of complex features in the vertebrate nervous system, with special emphasis on the evolutionary origin of placodes and neural crest, and postulate a pre-patterned migratory pathway of cells, which, in the epidermis, may represent an intermediate state towards the deployment of one of the most striking innovative features of vertebrates: the neural crest and its derivatives.

Tob is a potential marker gene for the basal layer of the epidermis and is stably expressed in human primary keratinocytes

BACKGROUND

Epidermis consists of multiple layers, from the proliferating basal layer to terminal differentiated cornified layers, and these layers are defined by differentiation status. Tob gene product is known to be a member of the BTG antiproliferative protein family. We investigated the expression pattern of Tob gene product to understand the possible role in differentiation of keratinocytes and epidermis.

OBJECTIVES

In this study, we examined the expression of Tob gene product in the primary cultured human keratinocytes and in the in vivo epidermis.

METHODS

The expression of Tob gene product was assessed by Western blotting analysis. Cellular localization of Tob was detected using the green fluorescent protein-tagged Tob cDNA expression construct. In vivo expression of Tob gene product in the epidermis was determined by immunohistochemistry with paraffin sections.

RESULTS

Tob family members are degraded by the ubiquitine-proteasome system triggered by the growth signal. Tob is stably and abundantly expressed in primary cultured human keratinocytes. Furthermore, the expression of Tob in the keratinocytes persists during the differentiation induced by calcium; however, it was not detected in primary cultured fibroblasts. Also, the subcellular localization of Tob is mainly in the cellular membrane in the primary human keratinocytes. We evaluated Tob expression in normal skin, oral mucosa and different diseases, such as psoriasis, X-linked ichthyosis and squamous cell carcinoma (SCC). Using immunohistochemical analysis, we observed that Tob was selectively expressed in the basal layer of X-linked ichythyosis and the hyperproliferative basal layer of psoriasis and oral mucosa as well as in normal epidermis. In SCC, the expression of Tob gene product was relatively decreased.

CONCLUSIONS

Tob is stably expressed in primary human keratinocytes and it is specifically expressed in the basal layer of in vivo epidermis.

Regulation of subcellular localization of the antiproliferative protein Tob by its nuclear export signal and bipartite nuclear localization signal sequences

Tob, a member of the Tob and BTG antiproliferative protein family, plays an important role in many cellular processes including cell proliferation. In this study, we have addressed molecular mechanisms regulating subcellular localization of Tob. Treatment with leptomycin B, an inhibitor of nuclear export signal (NES) receptor, resulted in a change in subcellular distribution of Tob from its pan-cellular distribution to nuclear accumulation, indicating the existence of NES in Tob. Our results have then identified an N-terminal region (residues 2-14) of Tob as a functional NES. They have also shown that Tob has a functional, bipartite nuclear localization signal (NLS) in residues 18-40. Thus, Tob is shuttling between the nucleus and the cytoplasm by its NES and NLS. To examine a possible relationship between subcellular distribution of Tob and its function, we exogenously added a strong NLS sequence or a strong NES sequence or both to Tob. The obtained results have demonstrated that the strong NLS-added Tob has a much weaker activity to inhibit cell cycle progression from G0/G1 to S phase. These results suggest that cytoplasmic localization or nucleocytoplasmic shuttling is important for the antiproliferative function of Tob.

Identification of the Anti-proliferative protein Tob as a MAPK substrate

Mitogen-activated protein kinases (MAPKs) regulate a wide variety of cellular functions by phosphorylating their specific substrates. Here we have identified Tob as a novel substrate of MAPK. Tob, a member of the Tob and B-cell translocation gene anti-proliferative protein family, is shown to negatively regulate the proliferation of osteoblasts and T cells. In this study, our two-hybrid screening has identified Tob as an ERK2-interacting protein. Biochemical analyses have then shown that ERK MAPK (ERK2) and JNK/SAPK (JNK2) bind to and phosphorylate Tob in vitro. ERK catalyzes the phosphorylation more efficiently than JNK. When the ERK pathway is activated in cells, phosphorylation of Tob is induced. An ERK-binding or -docking site locates in the N-terminal portion of Tob, and phosphorylation sites reside in the C-terminal stretch region. The docking is crucial for efficient phosphorylation. Mutant forms of Tob, in which serines are replaced by glutamic acids to mimic phosphorylation, show a much reduced ability to inhibit the cell cycle progression to S phase from G(0)/G(1) phase, as compared with wild-type Tob, indicating that ERK phosphorylation negatively regulates the anti-proliferative function of Tob.

Were vertebrates octoploid?

It has long been suggested that gene and genome duplication play important roles in the evolution of organismal complexity. For example, work by Ohno proposed that two rounds of whole genome doubling (tetraploidy) occurred during the evolution of vertebrates: the extra genes permitting an increase in physiological and anatomical complexity. Several modifications of this 'two tetraploidies' hypothesis have been proposed, taking into account accumulating data, and there is wide acceptance of the basic scheme. In the past few years, however, several authors have raised doubts, citing lack of direct support or even evidence to the contrary. Here, we review the evidence for and against the occurrence of tetraploidies in early vertebrate evolution, and present a new compilation of molecular phylogenetic data for amphioxus. We argue that evidence in favour of tetraploidy, based primarily on genome and gene family analyses, is strong. Furthermore, we show that two observations used as evidence against genome duplication are in fact compatible with the hypothesis: but only if the genome doubling occurred by two closely spaced sequential rounds of autotetraploidy. We propose that early vertebrates passed through an autoautooctoploid phase in the evolution of their genomes.

Genes modulated by histone acetylation as new effectors of butyrate activity

A wealth of evidence correlates the chemopreventive activity of a fiber-rich diet with the production of butyrate. In order to identify the genes transcriptionally modulated by the molecule, we analyzed the expression profile of butyrate-treated colon cancer cells by means of cDNA expression arrays. Moreover, the effect of trichostatin A, a specific histone deacetylase inhibitor, was studied. A superimposable group of 23 genes out of 588 investigated is modulated by both butyrate and trichostatin A. Among them, a major target was tob-1, a gene involved in the control of cell cycle. tob-1 is also up-regulated by butyrate in a neuroblastoma-derived cell line, and its overexpression in the colon cells caused growth arrest. Our findings represent an extensive analysis of genes modulated by butyrate and identify completely new effectors of its biological activities.

In search of a function for the TIS21/PC3/BTG1/TOB family

The Btg family of anti-proliferative gene products includes Pc 3/Tis 21/Btg 2, Btg 1, Tob, Tob2, Ana/Btg3, Pc3k and others. These proteins are characterized by similarities in their amino-terminal region: the Btg1 homology domain. However, the pleiotropic nature of these family proteins has been observed and no common physiological function among family members was suggested from the history of their identification. Recent progress in the search for Btg family functions has come from the analysis of cell regulation and of cell differentiation. It is now emerging that every member of this family has a potential to regulate cell growth. We would like to propose here to use a nomenclature APRO as a new term for the family.

The gene PC3(TIS21/BTG2), prototype member of the PC3/BTG/TOB family: regulator in control of cell growth, differentiation, and DNA repair?

PC3(TIS21/BTG2) is the founding member of a family of genes endowed with antiproliferative properties, namely BTG1, ANA/BTG3, PC3B, TOB, and TOB2. PC3 was originally isolated as a gene induced by nerve growth factor during neuronal differentiation of rat PC12 cells, or by TPA in NIH3T3 cells (named TIS21), and is a marker for neuronal birth in vivo. This and other findings suggested its implication in the process of neurogenesis as mediator of the growth arrest before differentiation. Remarkably, its human homolog, named BTG2, was shown to be p53-inducible, in conditions of genotoxic damage. PC3(TIS21/BTG2) impairs G(1)-S progression, either by a Rb-dependent pathway through inhibition of cyclin D1 transcription, or in a Rb-independent fashion by cyclin E downregulation. PC3(TIS21/BTG2) might also control the G(2) checkpoint. Furthermore, PC3(TIS21/BTG2) interacts with carbon catabolite repressor protein-associated factor 1 (CAF-1), a molecule that associates to the yeast transcriptional complex CCR4 and might influence cell cycle, with the transcription factor Hoxb9, and with the protein-arginine methyltransferase 1, that might control transcription through histone methylation. Current evidence suggests a physiological role of PC3(TIS21/BTG2) in the control of cell cycle arrest following DNA damage and other types of cellular stress, or before differentiation of the neuron and other cell types. The molecular function of PC3(TIS21/BTG2) is still unknown, but its ability to modulate cyclin D1 transcription, or to synergize with the transcription factor Hoxb9, suggests that it behaves as a transcriptional co-regulator.

Cloning and characterization of the mouse tob2 gene

Human Tob2 is a member of the Tob/BTG1 anti-proliferative family of proteins. Here, we report the molecular cloning and characterization of the mouse tob2 gene. The tob2 gene contains an open reading frame of 345 amino acids with an 89% identity to its human counterparts. The coding region of mouse tob2 is not interrupted by introns. The tob2 transcript is 4.2kb long, the size being similar to that of the human tob2 transcript, and detected ubiquitously in various tissues of adult mice. In addition, in situ hybridization shows that tob2 is ubiquitously expressed in embryo, the level of expression being especially high in skeletal muscle. Collectively, Tob2 is suggested to play roles both during embryogenesis and in adults.

A novel member of the tob family of proteins controls sexual fate in Caenorhabditis elegans germ cells

Although many cell fates differ between males and females, probably the most ancient type of sexual dimorphism is the decision of germ cells to develop as sperm or as oocytes. Genetic analyses of Caenorhabditis elegans suggest that fog-3 might directly control this decision. We used transformation rescue to clone the fog-3 gene and show that it produces a single major transcript of approximately 1150 nucleotides. This transcript is predicted to encode a protein of 263 amino acids. One mutation causes a frame shift at the sixth codon and is thus likely to define the null phenotype of fog-3. Although the carboxyl-terminus of FOG-3 is novel, the amino-terminal domain is similar to that of the Tob, BTG1, and BTG2 proteins from vertebrates, which might suppress proliferation or promote differentiation. This domain is essential for FOG-3 activity, since six of eight missense mutations map to this region. Furthermore, this domain of BTG1 and BTG2 interacts with a transcriptional regulatory complex that has been conserved in all eukaryotes. Thus, one possibility is that FOG-3 controls transcription of genes required for germ cells to initiate spermatogenesis rather than oogenesis. This model implies that FOG-3 is required throughout an animal's life for germ cells to initiate spermatogenesis. We used RNA-mediated interference to demonstrate that fog-3 is indeed required continuously, which is consistent with this model.

Interaction of BTG1 and p53-regulated BTG2 gene products with mCaf1, the murine homolog of a component of the yeast CCR4 transcriptional regulatory complex

Both BTG1 and BTG2 are involved in cell-growth control. BTG2 expression is regulated by p53, and its inactivation in embryonic stem cells leads to the disruption of DNA damage-induced G2/M cell-cycle arrest. In order to investigate the mechanism underlying Btg-mediated functions, we looked for possible functional partners of Btg1 and Btg2. Using yeast two-hybrid screening, protein-binding assays, and transient transfection assays in HeLa cells, we demonstrated the physical in vitro and in vivo interaction of both Btg1 and Btg2 with the mouse protein mCaf1 (i.e. mouse CCR4-associated factor 1). mCaf1 was identified through its interaction with the CCR4 protein, a component of a general transcription multisubunit complex, which, in yeast, regulates the expression of different genes involved in cell-cycle regulation and progression. These data suggest that Btg proteins, through their association with mCaf1, may participate, either directly or indirectly, in the transcriptional regulation of the genes involved in the control of the cell cycle. Finally, we found that box B, one of two conserved domains which define the Btg family, plays a functional role, namely that it is essential to the Btg-mCaf1 interaction.